In the field of micro-electronics, it may be desirable to reduce the electrical resistance of the layers of a substrate intended for making electronic components. This reduction in resistance may be obtained by increasing the concentration of the carriers.
This increase in the concentration of carriers, the main steps of which are schematically illustrated in FIG. 1, is obtained in a conventional way by doping the substrate 1, illustrated in FIG. 1a. In silicon this may be accomplished by implanting a dopant species 2, with reference to FIG. 1b. The implantation is typically performed with dopant species such as, for example, phosphorous or boron. The substrate obtained according to such a known method, with reference to FIG. 1c, thus comprises an upper doped area 3 and a lower crystalline area 4.
However, the dopant species has limiting solubility which corresponds to the maximum concentration of the carriers, which may be produced in the supporting substrate. Thus, it would be advantageous to be able to dope the substrate beyond standard solubility limits of the order of 1e20 at/cm3. For this purpose, applying a so-called solid phase epitaxy (SPE) is also known, the main steps of which are schematically illustrated in FIG. 2.
The solid phase epitaxy is illustrated in FIG. 2 for treating a supporting substrate 10 which is typically in silicon. During a first step (FIG. 2b), atomic species 11 such as silicon are implanted from the upper face of the supporting substrate in order to create an upper amorphous layer 12 in the supporting substrate 10, as illustrated in FIG. 2c. After implantation of atomic species, the substrate includes a lower crystalline layer 13 and an upper amorphous layer 12. It should be noted that this implantation of species 11 also generates a region 15 immediately underneath the amorphous layer 12 that contains a few atomic species (such as silicon) in interstitial positions without the structure of this region 15 being describable as amorphous. In a second optional step, with reference to FIGS. 2d and 2e, a doping species 14 such as phosphorous or boron is implanted in the amorphous layer 12.
Next, with reference to FIG. 2f, after the previous optional step for implanting a dopant species, low temperature recrystallization annealing is performed, the dopant species 14 implanted in the amorphous layer 12 being then activated so that the dopant species 14 is put in a substitutional position in the layer 12 in a large proportion.
By low temperature recrystallization annealing, is meant a heat treatment at a temperature between 550° C. and 650° C. which allows recrystallization of the amorphous layer 12 from the crystalline layer 13 of the substrate 10, this layer 13 playing a role of seed layer.
With such a technique, it is thereby possible to activate the dopants beyond their limiting solubility in the supporting substrate 10. It will be noted that, in the case of silicon, the limiting solubility of most current dopants varies between 1e18 and 1.5e20 at/cm3 for temperatures from 800° C. to 1,150° C. With this increase in the solubility of the dopants, the concentration of carriers in the supporting substrate 10 may be increased, thereby causing a lowering of the source/drain resistances of the electronic components made on the substrate and, consequently, low power consumption of the components.
This technique, however, is also associated with a drawback. End of range (EOR) type point defects are generated at the end of the recrystallization annealing, in the region 15 immediately located under the recrystallized layer 12. These EOR defects are caused by crystalline defects which appear during the amorphization step and which develop during the recrystallization step. The EOR defects considerably deteriorate electrical performances and more particularly the mobility of the carriers of the substrate 10 so that these substrates are unsuitable for making electronic components. Thus, the solid phase epitaxy type methods are associated with certain limitations, if the methods allow the limiting solubility of dopants to be increased in a substrate.
It is, therefore, desirable to have a method which would eliminate these limitations.